Photoamplifier



Jul 25, 1950 R. o. BRADLEY 2,516,179

PHOTO AMPLIFIER Filed Oct. 2, 1948 2 Sheets-Sheet l 47 VOEFTS E (REDUCED SCALE) 0 TlME INCREASE L. IGHT '//mH/L48 T Eg7 43% l M 50 V DECREASE UGHT jig I INVENTOR.

Robert O fi/"czd/e v July 25, 1950 R. o. BRADLEY 2,516,179

PHOTO AMPLIFIER Filed Oct. 2, 1948 2 Sheets-Sheet 2 IN V EN TOR.

Robe/"Z 0. firad/ey Patented July 25, 1950 PHOTOAMPLIFIER Robert. Bradley, Toledo, 0hio,.assignor to T0- ledo S'cale Company, Toledo, Ohio, a corporation of New'Jersey ApplicatiOnOctoberZ, 1948, Serial No. 52,445 3'Clai'ms. (o1. 250 '214)' This invention relatesto electronioamplifiers.

cording to the quantity of lightincident. upon. a.

photoelectric cell.

Many electronic amplifiers. have been con-. structedfor use with photoelectric cells. Ingeneral, these amplifiers have. suffered from oneor more defects which have limitedtheir. field of usefulness as controlv elements. Among these defects is the fact that the output current of the amplifier variesa proportionally to the light inci-' dent upon the photoelectric celland thus promotesa chattering. operation'of. the. relay as the light intensity approaches or recedes. from that intensity at, which the relay operates. Another defect is the sensitivity ofisuch: amplifiers to extraneous voltages that. may be. inducedin. the leads connecting. the photoelectric: cell. to the. amplifier.

The principal object of thisuinvention is. the

provision of an electronic. amplifier: the output current. of which is. substantially zero until a selected light intensity. is reached at whichtimethe output current suddenly rises tc-substantially its maximum value and remains at. such value regardlessv of. further increases. inthe. intensity of the-light incident. upon the photoelectric cell.

Another object. of the. invention is to utilize voltages induced in the leads connecting the photoelectric cellto the amplifier as-a means of. securing suddentransition fromzero current condition to. full currentcond'ition as the intensity of the light incident'upon the. photoelectric cell increases continuously from alow value to a high value.

More specific objects and. advantages are apparent from the following description in which reference is made to'the' accompanyingid'rawings.

According-to the invention, the-leads connecting a photoelectric cell to its amplifier are-carried in the same conduit'orcable as the leads supply ingcurrent to a bulb servingas a light source for the" photocell. The amplifierisconditioned so that the voltages induced" in the photoelectric cell-leads because of'their proximity to the light sourcelea'ds' are of suchphase and amplitude as to induce sudden changes in' the-"output of the amplifier as the input fromthe photoelectric cell changes in acontinuous manner:

An amplifier constructed to operate according to: the invention:isillustrated schematicallyin the accompanying drawings.

In thedrawingsr FigureTI is aschematiccircuit' diagram of the improved amplifier;

Figure II is a diagram in the nature of an oscillogram indicating the instantaneous voltages. on certain elements of the amplifier as functions. of time.

Figure III is a schematic diagram of a slightly modified. amplifier constructed according to the invention.

These specific figures andv the accompanying. description are intended merely to illustrate the. invention but not to impose limitations on the" claims.

The amplifier shown schematically in Figure I has a power transformer l of which a primary winding 2 is connected to a suitable source of power supply while alow voltage secondary winding 3 supplies filament power for the amplifier and light source. A high voltage secondary winding 4 supplies high voltage power for the ampliher. One side of the low voltage secondary wind-- ing"3.is.ccnnected to a groundedlead' 5 while the other terminal of the low voltage winding 3 isconnected to a lead 6. Current supplied from the low voltage winding 3 flows through the lead 6 and a branch lead I to a filament 8 of a rectifier tube 9 the other side of which is connected to the; grounded lead 5. Current also fiows from. thelead 5 through another branch lead Ill to a filament H of a grid controlled gas discharge tube 12 and back to the grounded lead 5. In addition current flows through a continuation of the lead 6 extending through a conduit and leading to afilament l3 of a light source bulb 14. The other side of the filament i3 is connected to the ground wire 5.

The high voltage secondary winding 4 has oneterminal grounded to the ground lead 5 and its other terminal connected through a lead [5, a relay coil IB, and lead ll to an anode [8 of the grid? controlled discharge tube I2. The discharge tube l2" has its cathode l9 and its screen grid 20 connected directly to the grounded lead 5 thus completing the anode circuit of the discharge tube I 2. Since, when the discharge tube i2 is conducting, the current flow through the anode circuit including the relay coil l6 consists of half cycle pulses, a condenser 2! is connected in parallel with the relay coil 16 to minimize or eliminate any chattering of an armature 22 cooperating with the relay coil lfi. The relay armature-22 controls a set of contacts 23 which may be connectedto control other equipment.

A- photoelectric cell 24 is arranged to receive light from the light source bulb M as long as a light intercepter 25 connected to a condition responsive apparatus or mechanism that initiates the control does intercept the light from the light The rectifier tube 9 operates as a half-Wave rec-' Its output, the voltage supplied to the tifier. lead 3|, is smoothed by a parallel combination of a resistor 33 and condenser 34 so that substantiallybonstant direct current voltage is maintained between the rectifier output lead 3| and the grounded lead 5. 4

The junction between the voltage divider resistors 21 and 28 is connected through a, disk type rectifier 35 and the parallel combination of a resistor 36 and condenser 3! to the grounded lead 5. The rectifier 35 is connected so that the potential at the junction between the rectifier 35 and the resistor 36, which junction is connected to a bias lead 38, is maintained at a negative voltage with respect to the grounded lead 5. The bias lead 38 is connected through high resistance resistors 39 and 40 and a lead 4| to a cathode 42 of the photoelectric cell 24. The junction between the resistors 39 and 40 is connected through a lead 43 to a control grid 44 of the gaseous discharge tube l2.

To minimize the efiects of transients, both" those that are transmitted from the power supply source through the primary winding 2 and those that result from the start of conduction of the gaseous discharge tube l2, a small condenser 45 is connected across the high voltage secondary winding 4.

' In this circuit as long as no light from the light source [3 is incident upon the photocell 24 the resistance to current flow of the photoelectric cell 24 is much greater than the resistance of the resistors 39 and 40. As a result, the potential of the control grid 44 is maintained at a sulficiently negative potential that no current may flow through the gas discharge tube I2 and,

therefore, no current flows through the relay coil l6. As the light intercepter 25 moves away from the light path so that'light reaches the photocell 24 its resistance to current flow decreases and the resulting increase in current flowing through the resistors 39 raises the potential of the control grid 44 until finally current fiow through the discharge tube [2 is no longer prevented and the relay is thereupon energized.

A gas discharge tube, such as the tube I2, has the characteristic that once current flow is established between its anode and cathode its control grid no longer exercises control Of the flow of current. The anode current flow may be interrupted only by reducing the anode potential with respect to the cathode to zero or negative values. In'this circuit the anode I8 is supplied without rectification from the high voltage winding 4 of the transformer. It is thus supplied with alternating voltage and for a half cycle of each complete cycle is driven negative with respect to the cathode so as to interrupt the anode current flow and permit the control grid 44 to regain control should its potential increase in a negative direction so as to call for zero current flow through the tube.

It is desirable in the interest of securing the best operating conditions for the relay that the current flow through the relay be substantially zero until such time as it is required to 6 056 and that then the current should increase suddenly to its maximum value or substantially its maximum value and remain at such value until the relay is to be opened. This is accomplished in this amplifier since the lead 4| the potential of which varies according to the current flow throughth'e photoelectric "cell "24" isincluded in the same conduit "as thelead 6 supplying alternating current to the light source bulb l4. Since the lead 6 carries alternating current voltage and is'in proximity to the lead 4!, the stray capacity between these. leads indicated by the dotted condensers 46 induces an alternating current voltage inthe leadQMQwhich voltage leads in phase the voltage output of the high voltage winding 4.

This capacitivelyin'duced voltage serves to raise .the grid; potential of the control grid 44 during the early 'part of each -.cycle and to reduce the voltage duringthe middle and latter part ofthe cycle. This 'efie'ct' is great enough so that the grid 'controlle'dtube l 2fires (establish anode current n wrnear' the start of the'positive half cycle of the anode voltage if it is't o be established at all during that half cycle. Therefore, the dis charge tube l2 either passes nocurrent or passes current during substantially all of the half cycle.

The phase relationships and magnitudes of the various voltages existing" in 'the' amplifier and contributing to this result are shown diagram--' matically in Figure II," In this figure a sinecurve 4'lrepresent'sthe voltag'e applied to the anode l8- of the discharge tube -l2 :as'lon'g as no current flows through the tube I2.

reduced scale with respect to the other curves of rent will flow even though a positive plate potentialexi'sts. Theinstant'aneous gridpotential required to prevent anode current flowv is indicated: by a dotted line 48. The potential applied to the control grid 44 isindicated by a sine curve 49 the axis 50 of which is {displaced negatively with respect to the z ero' volta'ge axis or the anode cuiw rentvoltage curve'4'i. ,The'D. C. voltage applied to the grid 44a nd represented in Figure II by the distance between thezero voltage axis and the axis 50 of theeurveAQ is determined by the amount of light incident on the photoelectric cell 24. Aslon fas thereis no light the grid isbiased negatively according to the voltage developed bythe dry disk rectifier 35. As the light on. the photoelectric cell 24 isincreased the average potential ofthe control grid 44'. rises andv may actually become positive with respect to thepo tential of the ground lead .5 represented by the zero voltage axis of Figure II. The alternating component of grid voltage represented by the sine curve 49 results from the voltage induced by the capacity between-the leads 6 and 4|. This alter nating component remains substantially constant and is displaced in phase ahead of the anode voltage wave 41 by an amountdetermined bythe relative magnitude of the resistances 39 and 40 and the interlead capacitances 46.

As the light incident on the photoelectric'cell 24 increases and the distance between the zero voltage axis and the average grid potential rep-- resented by the line 50 decreases'a point is finally reached at which the sine curve" 49 becomes tangent to orintersect the-dotted curve 48 representing the critical grid potential; When these curves become tangent or cross, anode current is I The ordinates of this curve 4! represent the instantaneous voltages; applied to the tube and the curve is drawn at a amen-cw established. through thegrld controlled discharge tube l2; From the" relative shapeaof. the curves shown in" Figure 'II it may be seen" that the curves' firsttouch' each other soon 'after th'e -anodevolt age becomes positive and long before it reaches its maximum value. In this a'rrangement the' anode'current through the t'ubeis zero until the curves become tangent 1 and then itsuddenly- 111- creasestoa value represented by'the shaded area" between the anode voltage curve H and the -zero axis.- Anyfurther' increase in"light'onthe photo electric: cell' causes a-slight increasein anodecu-r rent since the first intersection of the curves 48 and 49' occurs somewhat earlier in the cycle.

This arrangement provides substantiallysnap action" insofar as the relay" coil [6 is concerned because the current fibw through the relay: coil" is" either zero or substantially equal tothemaxi mum value that will'fiow underany conditions;

Without the effect of the capacity between the leads 6- and Al thegrid potentiairepresented by the'line 50 as the light is increasihgflrstbecomes tangent tothe-critical" grid potentialcurve-48* near the middle of" the positive half cycle of the" anode'voltage'wave 41. Under this condition the current flowthrough the-discharge tube changes sudd'enlyfrom zero to a value equal to-h'alfth'emaximum value of the current that flows when full light intensity'i's reached. To-prevent chattering of the relay this'means that the" relay must definitelyclose'on half ofits ratedcurrent: This requires that the-relay coilqbe able' to dissipate' under operative conditions" four times-the power that isava-ilable whenthe discharge tube first begins to conduct current.

The amount of capacity introducedwithinthe'" cable or externally between the-leads" 6" and M is not critical. Too" small -a capacitance which may result if the cable. or conduit is extremely short may result in an" inadequate alternating voltage component" in the: grid. control voltage,

i. e., the alternating component representediby the curve 49 would. be insufllcient to. matchthe curvature of the critical .gridi voltage curve so as to advance the tangent point between these." curves ahead of the midpoint of the positive half" cycle. If this condition is observed a small additional condenser may be connected between the leads to compensate for the shortness of the conduit.

The presence of extremely long leads or a long conduit increases the capacity between the leads 6 and 4|. The only effect of this increase is to increase the amplitude of the alternating component of grid voltage and reduce its phase lead. The reduction in phase lead, however, is not serious since the impedance of the interlead capacitance must become small as compared to the resistance of the resistors 39 and 40 before a substantial loss in circuit efficiency occurs.

Figure III illustrates a possible modification of the amplifier circuit shown in Figure I. This circuit differs from that shown in Figure I in that the thermionic rectifier is substituted for the dry disk rectifier and that a set of normally open contacts controlled by external timing equipment is included in the anode circuit of the gas discharge tube to prevent anode current flow before the cathode of the tube has reached operating temperature.

In other respects the circuits are similar. Thus the circuit shown in Figure III has a power transformer 5| the primary winding 52 of which is connected to a suitable source of power and the secondary windings 53 and 54 of which supply 51 of a dual rectifier 58, a filament 59 of agasdischargetube B0 and a filament 61 of alight source bulb 62: are connected in parallel between the: groundedlead 55 -and the low voltage powerlead The'high voltage winding 54has'one terminal" connected to' the grounded lead 55' and. its other: terminal connected to'supply current through a; mate, a coil 64 of'a relay65, alead 66, normallyopen contacts 6'! of a time-delay relay; otherwise not shown, anda lead 68 connected to-an anode;

The dis-- 6 9 of the gaseous discharge tube 60. charge tube 60' has its cathode l0 and itsscreen' grid 'Hconnected directly to the grounded lead. 55; Since the anodecurrent flow through the gasdischarge tube 58 consists of pulses of current acondenser 72. is connected in parallel withthe relay coil 64 to smooth out the fiuctuating'cun rentandprevent chattering of the relay. The

relay 65..has contacts 13 that may be connected to electricallycontrolled auxiliary equipment.

A photoelectric cell 74 that receives light from;

the light source 62 as long as an intercepter 15 does not cut ofi the light. has its anode 16 con- ,nected through a lead TI to a cathode 18 of the rectifier tube 58. The cathode 18 cooperates with an anode 19 that is connected to the junction between resistors 89 and 8! of a voltage. divider connected across the high voltage winding. 54.

This rectifier section including the anode 19 and cathode 18 operates as a half-wave rectifier and a filter comprising a resistor 82 and 83 is connected between the lead TI and the grounded lead 55*to maintain substantially constant voltage. onv the lead 11.

The voltage divider also includes a resistor 84. and the junction between the resistors8l and 84 is connected to a cathode 85 of a second section of the rectifier tube 58. An anode 86 cooperating with the cathode 85 is connected to a bias lead 81 and also through a parallel combination of a resistor 88 and condenser 89 to the grounded lead 55. This last described rectifier section serves to maintain bias voltage, i. e., negative voltage, on the lead 81. The lead 81 is connected through high resistance resistors 90 and 9| and a lead 92 to a cathode 93 of the photoelectric cell 74. Furthermore, the midpoint of junction between the resistors 90 and 9! is connected to a control grid 94 of the gas discharge tube 60. As in the preceding circuit the leads 56 and 92 along with the leads 55 and 11 are carried through a conduit and the interlead capacitance between the leads 56 and 92, represented by the dotted condenser 95, supplies an alternating current component to the grid voltage applied to the control grid 94 so that current flow through the gaseous discharge tube 69 is initiated near the start of the positive half cycle if any current is to how during that half cycle.

This circuit differs from the one first described in that a second section of a thermionic rectifier tube is employed to supply the grid bias for the gaseous discharge tube. Such a rectifier tube is not instant start, i. e., it does not supply bias voltage until its cathode has reached operating temperature. Therefore, there is a time interval immediately after the power transformer is energized during which the gaseous discharge tube is operating without grid bias. It is also necessary, in "order to protect the emitting surface ofthe cathode of the gaseous discharge tubev to ensure that the cathode is at operating temperature before current is allowed flow. In the circuit first described, the dry disk rectifier which is instant starting supplies grid bias to prevent conduction through the tube. This condition ob-' tains even though the photocell may receive a 6 and these contacts 6'! are controlled by a time-' delay relay so that the contacts are closed after the cathode of the gas discharge tube 60 has reached normal operating temperature.

' This circuit has all of the advantages of the one'first described except for the necessity of opening the anode circuit of the gaseous discharge tube during the initial warming up period of the cathodes of the tubes. Each circuit employs a capacitive cooperation between two of its leads either wholly or in part to provide an alternating component of grid voltage that produces a substantially snap action effect in the output circuit of the amplifier as the amplifier input is slowly varied.

Various modifications of the circuit detail may be made without departing from the spirit and scope of the invention.

' Having described the invention, I claim:

. 1. In a relay circuit of the class described, in

combination, a grid controlled gaseous discharge tube, a 'relay that is energized by current flow through the tube, an alternating current source of p,ow er for the discharge tube, a photoelectric cell, adirect current voltage supply for the photo electric cell, circuit means for connecting the photoelectric. cell to the grid of the discharge tube,

alight source for the photoelectric cell, a lead.

carryingalternating cur cut for energizing the: light source, said lead being located adjacent said circuit means whereby an alternating voltage is induced in said circuit means the phase of which leads the phase of the voltage applied to the anode of the discharge tube.

2. Ina relay circuit of the class described, in combination,--a grid controlled gaseous discharge tube, a relay that is energized by current flow. through thetube, an alternating current source of power for the discharge tube, a photoelectric,

cell,- a direct current voltage supply for the photocell, circuit .meansfor connecting the photoelectric cellato they-grid of the discharge tube and to the voltage supply, a;.-lightsource for the photoelectric cell, a lead carrying alternating current for energizing the light source, said lead and said circuit means being cabled together whereby an;

alternating voltage is induced in said circuit means; 1

-3. In a relay control grid and an anode, a relay that is connected in circuit-with the anode of the tube, a light source, an alternating current source of power. for the anode circuit of the tube and. the light source, a photoelectric cell, a direct current source of voltage forthe .cell, said cell and light source being remote from the discharge tube, and

acablehaving a pluralityof conductors serving toconnect the photocell to the grid of the discharge tube and the light source to its source of power, the conductor'to the light source serving by-its presence in the cable to also induce an. alternating voltagein the conductor to the photo-- cell effective to control the point of discharge of the discharge tube. I

ROBERT O. BRADLEY.

REFERENCES CITED 7 The, following references are of record in the file of this patent:

Gullik s'en and Vedderi Industrial Electronics,

1935 (John Wiley'and Sons pub), pages and circuit of the class described, in. combination, a gaseous discharge tube having a 

